1. Field of the Invention
The present invention relates to a pipette comprising a frame part with a cylinder formed inside the frame, a piston which moves within the cylinder and which has been gasketed in relation to the cylinder, a helical spring which keeps the piston at the upper position, and a filling press-button which affects the piston and by means of which the piston can be pressed down towards the tip of the pipette against the force of the helical spring to a depth determined by limiting members.
2. Description of the Prior Art
In a pipette of this type the limiting members determine the quantity of liquid taken into the pipette, and they can either be fixed at the manufacturing stage of the pipette or, alternatively, they are adjustable. In order to ensure that, when the button is pressed a second time, the pipette is definitely emptied entirely, it is known to provide it with two helical springs the rigidities of which differ from each other to such a degree that, when the button is pressed, a definite change is felt the more rigid spring begins to exert its effect. In this case, for filling the pipette, only the more flexible spring is compressed to a predetermined depth, where the more rigid spring begins to exert its effect, whereas, when the pipette is emptied, the button is pressed down all the way so that first the more flexible and then the more rigid spring are compressed.
This known arrangement has disadvantages, on the one hand in its imprecision, since the first "threshold" is not determined with absolute precision, and on the other hand in that it is difficult to use because, when the pipette is being emptied it is necessary to press the knob quite hard, which is strenuous for the user. Some kind of compromise must be made between these disadvantages, since if one is decreased the other one increases.